1. Field of the Invention
The present invention relates to a method of obtaining a monochromatic image from a color original by means of a color image recorder operable to read the image of the color original through image scanning thereof.
2. Description of the Prior Art
As is well known in the art, a color process scanner is employed for obtaining color printing blocks such as yellow, magenta, cyan and black blocks. The color process scanner photoelectrically scans an image of a color original and records respective color components of the image on respective recording medium. In a drum type scanner, for example, the recording medium, such as a photosensitive film, is wound on a recording drum, and the color component images are recorded thereon in parallel. The color component images are arrayed along the main scanning direction, (i.e., the circumferential direction) of the recording drum. In order to prevent a ripple effect on a printed image, the color component images are recorded as halftone dot images having different screen angles. For example, the respective screen angles for yellow, magenta, cyan and black blocks may be set at 0.degree., 15.degree., 75.degree. and 45.degree. respectively. A technique for attaining such an improvement has been established.
After the color component blocks are obtained, color inks corresponding to the color components are applied to the blocks, respectively, and color component images on the blocks are printed on a common paper through transferance of the inks, to obtain a printed color image.
On the other hand, it is often required to print a color image in a monochromatic color. In one basic method of attaining a monochromatic print, one of the color component blocks, e.g., the magenta block, is selected, and the monochromatic image is obtained by applying a black ink to the block to transfer the "black" image thereon to a printing paper. Although the method has the advantage that no complex process is required, the resultant monochromatic image lacks depth since the optical density range therein is narrowly restricted due to the intrinsic density range of the black ink.
To compensate for the lack of depth, another technique has been developed, wherein one of the color component signals obtained by reading a color original image is arbitrarily selected. The selected signal, e.g., a color component signal for a magenta block, is used to serially obtain recorded images at different screen angles. Then, a plurality of blocks are produced according to the recorded images, respectively, and different color inks (such as yellow, magenta and cyan) are applied to the respective blocks, and the blocks are used to print a single image. Although the printed image is formed by the combination of the different color inks, it is substantially monochromatic, since the color inks are so overprinted on the paper that the subtractive color mixture of the color inks expresses the monochromatic color, e.g., achromatic color. The technique is very useful for obtaining a monochromatic image having great depth, and therefore, it is often used in the printing process.
However, the conventional technique for attaining the overprinting has the disadvantage that the process is complicated and slow since the process step required to read the color original image must be repeated in order to obtain the different color blocks, and a desired gradation character and a screen angle must be provided for all of the color component signals serially while changing the screen angle for each color component signal. This is a serious disadvangage especially in the case where the image data processing is carried out with a color process scanner because the image processing takes a long time, and no other operations can be performed in the scanner until the image processing is completed.
Furthermore, the optical density of the recorded image is often shifted from the desired one for each color component, in which the shift depends on the selection of a color component signal to be employed, and therefore, the printed image is often somewhat unnatural.